1. Field of the Invention
The present invention relates generally to detection methods for cardiac arrhythmias, and more particularly, to methods of improved accuracy for identifying the presence of conditions that can be ameliorated by antitachycardia pacing, by cardioversion shock, or by defibrillation shock.
2. Description of the Prior Art
Fibrillation is a chaotic and uncoordinated contraction of the ventricular myocardium. The heart can be caused to return to normal rhythm by the application of an energetic electrical shock, typically of the order of 10 joules. Cardioversion involves the application of a more modest shock, typically in the neighborhood of one joule, and is appropriate in the case of ventricular tachycardia. The heart in this condition has a high, but well-defined rate, but its contraction is not as well-coordinated as it is in the normal state, the condition of normal sinus rhythm. Furthermore, if the rate is high enough, pumping efficiency declines significantly. Ventricular tachycardia, still further, is sometimes a precursor of fibrillation.
When the heart races, but retains well-coordinated contractions, effective intervention is sometimes possible by applying a pacing signal of still higher rate that "takes over" from the tachycardia, and then returns the rate to the proper range. This treatment is appropriate when the rise in rate has not been too large.
Several prior-art tactics have been used to identify malfunctions such as these, in order to apply the appropriate electrical protocol. But these have left much to be desired in terms of accuracy and reliability. For example, a simple and common criterion for the application of a defibrillation shock is a pulse rate above 170 per minute. But such an elevated rate can and should result from exercise, to name one cause, the case of "sinus" tachycardia. Under such conditions, a "false positive" instruction to the defibrillator results from the sole use of the simplistic rate criterion, followed by the delivery of a painful and possibly hazardous shock to the patient.
More sophisticated systems monitor more than one variable. For example, rate of change of the pulse rate is sometimes observed, to detect a rapid "jump" in heart rate. In variations on this method, one can monitor rate acceleration, or the magnitude of the rate change within a given interval. More general approaches are covered in the discussion below under the term "onset". Another criterion is duration of high heart rate, measured in the number of consecutive cycles above a specified rate. Other options examine the correlation between electrical pulses and separate signals derived from a mechanical sensor of some description.
Yet another option observes a quantity that is related to the waveform shape of the signal from the heart. For example, the ratio of the average absolute value of the signal voltage to the peak absolute voltage is small during normal sinus rhythm, because the signal departs from near zero for a relatively small fraction of the cycle. In the chaotic signal from a fibrillating heart it is nonzero much more of the time. This kind of absolute-amplitude observation is included below under the brief term "absolute-amplitude ratio" (AAR). The probability distribution function (PDF) is a useful detection approach involving waveform shape. A further criterion is signal amplitude. Amplitude declines significantly during fibrillation. Still further, one can observe blood pressure, blood pH, and other physiological-variable values. Finally, there is the stability criterion. Here the rate variation can suggest an exercise-related change in normal sinus rhythm, or the chaotic behavior connected with ventricular fibrillation, but rules out the middle ground of ventricular tachycardia.
A number of U.S. Patents deal with the detection of ventricular arrhythmias by using observations of more than one variable. The inventors, patents, dates, and variables can be summarized as follows:
Dennisten and Davis, U.S. Pat. No. 3,805,795, 1974, electrical and mechanical.
Langer and Heilman, U.S. Pat. No. 4,475,551, 1984, rate and PDF.
Imran, U.S. Pat. No. 4,796,620, 1989, rate and onset.
Chirife, U.S. Pat. No. 4,865,036, 1989, rate and mechanical.
Pless and Sweeney, U.S. Pat. No. 4,880,005, 1989, rate and stability.
In these multi-variable systems, it is standard practice to set a threshold level for each variable that, when exceeded, contributes a positive "vote" to the decision. These discrete criteria are combined in a logical AND function, thus requiring a unanimous vote, and a particular electrical treatment is initiated by the positive AND output.
To illustrate the shortcomings of such a system and also a single-variable threshold-method system, let us take a two-variable example. The same kind of example is useful to illustrate the advantages of the present invention, as described in the following section. The principles involved in more complex systems will be evident as extension of this example. Suppose that ventricular tachycardia is to be detected and at most two variables are to be monitored, heart rate and duration of the elevated rate.
As the single-variable example, let us take the case of pulse-rate monitoring only. This case is illustrated in FIG. 1A, where the x axis is labeled "rate". The y axis is present also, and is labeled "duration", but is included only to relate this case to subsequent cases. The standard technique chooses a discrete threshold r.sub..degree., and triggers electrical therapy when the rate exceeds r.sub..degree.. It follows, then, that any pulse rate falling in the shaded region will produce a positive indication. The obvious problem with this simplistic method is that even a condition represented by the point with the coordinates (r.sub.1, d.sub.1) will trigger therapeutic response in spite of the fact that the high rate was very fleeting.
Moving then to the two-variable method, we can see that such a false positive is ruled out by "ANDING" the pair of variables. It is now necessary to have r&gt;r.sub..degree., and d&gt;d.sub..degree. simultaneously in order to trigger electrical response. This situation is illustrated graphically in FIG. 1B, and the now more restricted shaded area represents the locus of points that trigger the system. Thus, it follows that the false-positive condition represented by the point (r.sub.1, d.sub.1) does not cause triggering. This is not the end of the story, however. Note that the condition represented by the point (r.sub.2, d.sub.2) will not trigger the system either, but certainly represents a case of tachycardia because the rate is high (though slightly below threshold) for a very long time. The system design of the present invention eliminates this shortcoming, as well as that associated with a point such as (r.sub.1, d.sub.1).